Example of why to use nested modules vs compact class namespaces in Ruby.

ruby-module-class-test.rb

module Wheels
  def diameter
    "diameter"
  end
end

class Autos::Car
  include Wheels
end
puts Autos::Car.new.diameter
# Results in: cars.rb:7:in `<main>': uninitialized constant Autos (NameError)

module Vehicles
  class Truck
    include Wheels
  end
end
puts Vehicles::Truck.new.diameter
# Results in: diameter

As a result of posting a comment on the Rubocop cop issue:
rubocop/rubocop#868 (comment)

This was posted as a reply:
https://twitter.com/damncabbage/status/463535409415204864

The uninitialized constant Autos error above occurs because the Autos namespace is not defined in advance of the Autos::Car class. It's a problem with the compact syntax, yes, but it's not what my tweet was whinging about.

The include Wheels business was about scoping; have a look:

module Autos
  module Wheels
    def diameter
      "diameter"
    end
  end
end

# ... later ...

class Autos::Car
  include Wheels
  # Kaboom: uninitialized constant Autos::Car::Wheels (NameError)
  # Doesn't have the "Auto" module context.
end

module Autos
  class Truck
    include Wheels
    # Is inside Autos, can find Autos::Wheel just fine.
  end
end

(I found this gist via rubocop/rubocop#868 when I went to look up the semantics again. I swear I didn't go googling my name. 😅 )